I want to start purchasing and selling my games on itch.io. This is my first experience on an online E-commerce platform.  I already have a bank account in my country - Nigeria but i don't have a paypal account yet. Do i need PayPal for payment?  and how do i setup the credit cards on itch.io?

And another problem is whether i'll need to pay taxes to Government where my purchases come from like in the EU or the United States. 

You'll an account with either PayPal or else Payoneer to receive the money. Also to fill in a tax interview, as indicated in your account settings. You only need to set up a credit card for buying, also in your account settings, under Payment -> Credit Card. See the Getting Paid FAQ for more details.

As for taxes, Itch can handle VAT and US sales tax if you pick the "Collected by Itch.io, paid later" mode, but you might still have to pay income tax in your own country on any money you receive; ask a local accountant to make sure.